well, actually, it's not that simple. you have hold the 1D a certain way to keep the door open. probably more pronounced coming from a D3 where the door is spring loaded. it's all about usability.
so unless you're used to a spring-loaded door, you won't miss that. huge design/engineering flaw.

They've added the spring-loaded door to the 5D MkIII, the last time I used a 1DMkIV I thought it also had a spring loaded door but maybe my memory is playing tricks on me. I guess I got spoiled using my MkIII =P